An electric motor-driven anti-lock braking system of the type to which this invention pertains is generally depicted in FIG. 1. Referring to FIG. 1, the braking system comprises a hydraulic boost unit 100, a wheel brake 102, an electric motor-driven hydraulic pressure modulator 104, and an electronic controller 106 for operating the modulator 104 with current from the vehicle storage battery 108. The boost unit 100 develops hydraulic pressure in line 120 in relation to the force applied to an operator-manipulated brake pedal, the line 120 being connected to the brake 102 via modulator 104 and brake line 122. Brake 102 is depicted as a disk brake caliper which develops braking force on the wheel rotor 126 in relation to the hydraulic pressure in brake line 122.
The modulator 104 comprises an actuator 130 axially displaceable in the modulator bore 132, a check ball 134 resiliently seated on a ball seat 136 disposed between the brake lines 120 and 122, and a bi-directional permanent field magnet DC motor 138 coupled to the actuator 130 via a reduction gearset 140 and a ball screw 142 to control the axial displacement of actuator 130.
Energization of the motor 138 is controlled by the electronic controller 106 in response to a signal on line 144 indicative of the angular velocity of rotor 126. When the controller 106 energizes the motor 138 for rotation in a forward direction, the ball screw 142 extends into the bore 132, thereby extending actuator 130 to unseat the check ball 134. This opens the communication between brake lines 120 and 122, and represents the normal or quiescent state of the anti-lock brake system. When the controller 106 energizes the motor 138 for rotation in the opposite, or reverse, direction, the ball screw 142 retracts actuator 130 within the bore 132, permitting spring 146 and the fluid pressure in brake line 120 to seat the check ball 134 on the ball seat 136, thereby isolating the brake line 122 from the brake line 120. In this condition, the brake fluid in line 122 backfills the modulator bore 132, relieving the fluid pressure developed at brake 102.
In anti-lock operation, the brake pressure in line 122 is modulated by repeatedly reversing the direction of rotation of motor 138 to effect a dithering movement of the actuator 130 in the bore 132. When an incipient wheel lock condition is detected, the controller 106 causes the motor 138 to rotate in the reverse direction to retract the actuator 130; when recovery of the wheel speed is detected, the controller 106 causes the motor 138 to rotate in the forward direction to extend the actuator 130 for increasing the brake pressure.
During the anti-lock operation described above, optimum braking performance requires different motor speed/torque characteristics depending on the direction of motor rotation. When the actuator 130 is being retracted (reverse direction of rotation), the torque requirement is relatively low, but the speed requirement is relatively high in order to enable quick relief of the brake pressure. When the actuator 130 is being extended (forward direction of rotation), the speed requirement is relatively low, but the torque requirement is relatively high in order to develop adequate pressure in brake line 122. Unfortunately, the speed/torque characteristics of a conventional DC electric motor are substantially the same in both directions, and some design compromises must be made in order to provide acceptable performance in both the forward and reverse directions of motor rotation. Of course, this involves some sacrifice in the anti-lock braking performance.
Directional speed/torque characteristics of permanent magnet field DC motors have been successfully varied through the use of auxiliary field coils wound around the stator permanent magnets and connected electrically in parallel with the windings of the rotor; see U.S. Pat. No. 5,000,524 to Savage, issued on Mar. 19, 1991, and assigned to the assignee of the present invention. In the forward direction of motor rotation, the auxiliary windings improve the motor torque characteristic by increasing the flux in the working air gap of the motor. In the reverse direction of motor rotation, the auxiliary windings improve the motor speed characteristic by decreasing the flux in the working air gap.
Some vehicular anti-lock braking systems are limited in the physical size of the motor housing and the magnitude of the motor current available. Without increasing the size of the housing, incorporating auxiliary field coils as noted above would require reducing the size of the permanent magnets to provide sufficient space around the periphery of the magnets for the field coils. Reducing the size of the permanent magnets would automatically improve the motor speed characteristic in the reverse direction of motor rotation by reducing the working air gap flux as a result of the loss of magnet material. However, the motor torque characteristic in the forward direction of motor rotation would be adversely impacted by the loss of the working air gap flux. Constructing a shunt coil large enough to supply the lost air gap flux would be possible were it not for the maximum motor current limitation. Torque is a function of both air gap flux and rotor winding current. The parallel configuration of a sufficiently sized auxiliary motor field coil significantly diminishes the current available for the rotor winding, thereby preventing the development of sufficient torque.